The disclosures of the following priority applications are herein incorporated by reference:
Japanese Patent Application No. 2000-119853 filed Apr. 20, 2000
Japanese Patent Application No. 2001-113513 filed Apr. 12, 2001
1. Field of the Invention
The present invention relates to a very-low-priced and high-zoom-ratio standard zoom lens system having lens construction of a negative-positive two-group type zoom lens system, or a three-group type zoom lens system having very simple lens construction similar to the aforesaid zoom lens system such as a negative-positive-negative type, a negative-positive-positive type, or the like, and relates to a photographic device equipped thereof
2. Description of Related Art
It has recently become common for a so-called standard zoom lens equipped with a single lens reflex camera to be used as a common-use lens instead of a standard lens. Accordingly, since this kind of zoom lens is always carried in a state equipped with a camera, it is necessary for the zoom lens to be light and compact as well as to have sufficient optical performance and cost performance. In order to satisfy the necessary condition, a two-group type zoom lens system consisting of a negative and a positive lens groups is the best choice, so that a lot of zoom lens system of this type have been proposed.
However, it has been thought to b difficult for a two-group type zoom lens system to satisfy both a large aperture ratio and a high zoom ratio. In order to realize especially a high zoom ratio, it is necessary to extremely increase the power of each lens group, which exists only two, within a practical range of a dimension and a diameter as a so-called standard zoom lens. Therefore, it has been the upper limit according to prior art to keep the zoom ratio about 2.8. Above all, a zoom lens disclosed in Japanese Laid-Open patent application Ser. No. 8-334694 applied by the same assignee of the present patent application has been known for realizing the highest possible zoom ratio of 2.83 as this type of compact zoom lens.
Moreover, in a standard zoom lens for a 35 mm film format, the range of variable focal length obtained by a two-group type zoom lens or a three-group type zoom lens (such as a negative-positive-negative type, a negative-positive-positive type, or the like) which is a modification of the two-group type zoom lens has been nominally about 2.8 such as 35-80 mm, 28-80 mm, or the like. Accordingly, it has been necessary for a zoom lens whose zoom ratio is three or over such as 28-105 mm to employ zoom lens construction of a four-group type or a five-group type. The four- or five-group type zoom lens naturally has complicated construction and increases the number of moving groups Moreover, the number of lens elements has become excessively large; for example, it has been generally 13-18 lens elements. Furthermore, the increase of the lens elements has caused the increase of the dimension and the weight. In addition, a technical hurdle of assembling technique has become high, and, finally, the all items have become factors of cost increase. Therefore, a four- or five-group type zoom lens has not been able to be provided cheaper than a two-group type.
In a zoom lens disclosed in Japanese Laid-Open Patent Application No. 8-334694, although the zoom lens is compact and has a small number of lens element, in order to satisfy both the zoom ratio of about 3.5 and the compactness and low cost, it has been necessary to develop optimum power arrangement and lens construction.
The present invention is made in view of the aforementioned problems and has an object to provide a two-group type zoom lens system consisting of a negative and a positive lens groups having an extremely high zoom ratio of about 3.5, which has been unique so far in any prior art, with very simple lens construction, compactness and a small diameter, and to provide a photographic device equipped with the zoom lens system.
According to one aspect of the present invention, a zoom lens system includes, in order from an object side, a first lens group having a negative refractive power, and a second lens group having a positive refractive power. Zooming is carried out by varying the space between the first lens group and the second lens group. The first lens group includes, in order from the object side, a negative lens group G1F composed of one or two-negative lens element, and a positive lens group G1R. The following conditional expressions are satisfied;
4.5xe2x89xa6|X2|xc2x7ft/fw2xe2x89xa615xe2x80x83xe2x80x83(1) 
xe2x80x830.7xe2x89xa6|f1|/(fwxc2x7ft)xc2xdxe2x89xa61.3xe2x80x83xe2x80x83(2)
where X2 denotes the maximum moving amount of the second lens group while zooming, f1 denotes the focal length of the first lens group, fw denotes the focal length of the zoom lens system in a wide-angle end state, and ft denotes the focal length of the zoom lens system in a telephoto end state.
In one preferred embodiment of the present invention, the following conditional expression is satisfied:
0.6xe2x89xa6f2/BFwxe2x89xa62xe2x80x83xe2x80x83(3) 
where f2 denotes the focal length of the second lens group, and BFw denotes the back focal length of the zoom lens system in the wide-angle end state.
In one preferred embodiment of the present invention, the second lens group further includes an aperture stop for defining an f-number of the zoom lens system, a positive lens group G2F including at least two positive lens element located to the object side of the aperture stop, and a negative lens group G2R including at least one negative lens element located to an image side of the aperture stop. The following conditional expression is satisfied:
xe2x88x921xe2x89xa6f2F/f2Rxe2x89xa6xe2x88x920.05xe2x80x83xe2x80x83(4) 
where f2F denotes the focal length of the positive lens group G2F, and f2R denotes the focal length of the negative lens group G2R.
In one preferred embodiment of the present invention, the negative lens group G1F in the first lens group includes at least one aspherical surface. The aspherical surface is defined by the following expression:
S(y)=(y2/R)/[1+(1xe2x88x92xcexaxc2x7y2/R2)xc2xd]+C3xc2x7|y|3+C4xc2x7y4+C5xc2x7|y|5+C6xc2x7y6+C8xc2x7y8+C10xc2x7y10+C12xc2x7y12+C14xc2x7y14 
where y denotes the height in a vertical direction relative to the optical axis, S(y) denotes displacement (sag amount) in the optical axis direction from the tangent plane on the vertex of the aspherical surface at the height y, R denotes a paraxial radius of curvature, xcexa denotes the conical coefficient, and Cn denotes n-th order aspherical surface coefficient. The conical coefficient xcexa satisfies the following conditional expression;
0xe2x89xa6xcexa less than 1xe2x80x83xe2x80x83(5). 
According to another aspect of the present invention, a photographic device is equipped with the zoom lens system described above.